Hygrometer

[Sarah]

An instrument for measuring the degree of moisture of theatmosphere. Daniell's hygrometer, a form of hygrometer consisting ofa bent glass tube terminating in two bulbs, the one covered withmuslin, the other of black glass, and containing ether and athermometer. Ether being poured on the muslin, the black ball, cooledby the evaporation of the ether within, is soon covered with dew; atthis moment, the inclosed thermometer gives the dew-point, and this,compared with the reading of one in the air, determines the humidity.

[Couch Potatoe]

a piece of equipment used to measure humidity (= how much water there is in the air)